lib: Add base64 library

Since base64 is such a simple and commonly used feature it makes no
sense to build the whole of mbedTLS for it. Instead take the
implementation that comes with mbedTLS and import it as a native library
outside of ext/ for all to use directly.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
5 files changed